The concept of high-altitude, long-endurance, solar-powered aircraft has been demonstrated by a number of aerial vehicle research projects. Solar power systems typically rely on an array of solar panels that interface with a battery grid (or similar battery systems) through control circuitry, such as a maximum power point tracker. A maximum power point tracker provides a circuit assembly that, in operation, adjusts the load impedance presented to the array of solar panels to achieve a maximum power out of the solar array. The power collected out of the solar array is then stored to the battery packs/assemblies of the battery grid.
A battery pack may employ battery cells of various chemistries. Lithium-polymer cells are higher in specific energy density per unit weight than most other battery chemistries; including nickel cadmium, lead acid, silver oxide, mercury, and alkaline dry cells. Furthermore, lithium-polymer cells have a higher voltage output per cell than many other systems; therefore, fewer cells are needed to achieve a given battery voltage.
Lithium-polymer cells are available in a variety of formats and housings, including prismatic cell battery packs and pouch cell batteries. Unlike prismatic cell battery packs that are typically rigid, pouch cell battery packs employ a flexible pouch with conductive foil tabs to carry the positive and negative terminals from the battery cells to the outside of the pouch. Pouch cell battery packs (or simply, pouch cells) typically experience a performance benefit when pressure is applied perpendicularly to the pouch cell's internal layers (e.g., compressing the cell stack). Preferably, this pressure should remain constant as the pouch cells swell cyclically.
A number of advancements have been made in such battery pressure management. For example, U.S. Pat. Nos. 5,670,272 and 6,040,085, each to Cheu et al., describe battery packaging having, inter alia, a gas spring positioned in a housing structure containing the battery cell assembly. Cheu et al. also describes injection of an expanding compound through a nozzle to the housing to exert pressure on the battery cell assembly. However, such metallic (or plastic) structural methods are excessively heavy (or overly complex) when configured to withstand the necessary applied load over time and, therefore, are not suitable for ultralight aircraft applications.
In view of the foregoing, a need exists for a lightweight battery assembly with a pressure management system or apparatus that can overcome the deficiencies of the prior art. Such a lightweight battery assembly may be employed with ultralight aircraft applications, such as long endurance solar-powered aircraft.